Multiple groove discharge lamp



1960 E. LEMMERS ETAL 2,950,410

MULTIPLE GROOVE DISCHARGE'L Filed May 12, 1958 Their A t t'orneg.

l. e hv- United States Patent MULTIPLE GROOVE DISCHARGE LAMP Eugene Lemmers and John 0. Aicher, Cleveland Heights,

Ohio, assignors to General Electric Company, a corporation of New York Filed May 12, 1958, Ser. No. 734,551

Claims. (31. 313-109 This invention relates to highly loaded low pressure discharge lamps having elongated tubular envelopes of noncircular grooved cross section. It is directed particularly to a highly loaded low pressure mercury vapor fluorescent discharge lamp and to a vitreous envelope of unique form and configuration especially suited therefor.

In copending application Serial No. 577,017 of Eugene Lemmers, filed April 9, 1956, entitled Tubular Electric Lamps and assigned to the same assignee as the instant invention, now Patent 2,915,664, various forms of reentrant cross section fluorescent lamps are disclosed. In one form which is sometimes referred to as a multiple or double groove lamp, short sections of groove are provided alternating on opposite sides of the envelope giving it a dimpled or crenelated appearance. The relatively short re-entrant grooves alternating on opposite sides combined with the slanting wall sections in between achieves a high ratio of perimeter to area of the cross section along with a relatively high implosion resistance. A high ratio of perimeter to area is highly advantageous in resonance radiation lamps, for instance in fluorescent lamps utilizing the resonance radiation of mercury vapor at 2537 A. to excite a phosphor coated internally on the walls of the envelope in order to produce visible light. It permits higher loadings and lumen output per unit axial length at a given efiiciency than heretofore possible.

Among the important factors involved in the high lumen efficiency achieved with a grooved contour is the high electron temperature or speed obtained simultaneously with reduced elastic collision losses due to more rapid diffusion of electrons, mercury ions, and radiation quanta to the bulb walls. For a given lamp length and wattage, the lamp current is less and the lamp voltage is higher than with a lamp of the same perimeter in a circular cross section, thereby reducing the proportion of both the cathode losses and the ballast losses.

In the form of non-circular fluorescent lamp commercially available from applicants assignee under the designation Power Groove, the longitudinal groove extends intermittently along one side and is interrupted at several places by regions of circular cross section. These regions increase the strength of the envelope and its resistance to implosion by reason of external atmospheric pressure. This specific form of lamp is disclosed and claimed in copending application Serial No. 578,772 of Eugene Lemmers et al. filed April 17, 1956, entitled Tubular Lamp Envelope, and likewise assigned to the same assignee as the present invention, now Patent 2,916,645. In this lamp, sometimes referred to as a single groove bridged lamp, the bridges or regions of circular section are electrically, and from the point of view of lumen eficiency, disadvantageous. It is estimated that in such a lamp, designated commercially 96 PG 17, of nominal length 8, diameter 2 /8", average lamp watts 200, lamp current 1500 milliamperes, lamp volts 160 and producing 13,000 lumens in the cool white color at 65 lumens 2,950,410 Patented Aug. 23, 1960 per watt, each bridge (8 in number) causes a loss of to lumens on a constant current basis. In other words, if the bridges could be eliminated and the-groove made continuous, the loading of the lamp could be increased to provide an additional output of 800 to 1200 lumens at the original efiiciency. However the bridge portions are necessary in order to provide suficient mechanical strength or implosion resistance in a bulb of acceptable wall thickness, for instance an implosion resistance of 90 pounds per square inch with a wall thick ness of .075".

The double-grooved or crenelated lamp of Lemmers application No. 577,017, new Patent No. 2,915,664, though somewhat more difiicult to manufacture, avoids the main disadvantage of the single-groove bridged lamp. The merging or oblique sections through which the dis charge or plasma channel is transferred from one side of the lamp to the other serve mechanically as substitutes for the bridges and in fact may be superior thereto. The electrical efiiciency of the discharge or the ratio of lumens produced to watts consumed in the merging sections is superior to that obtaining with a circular bridge section. In accordance with the present invention, we have found ways to further improve the electrical efiiciency by means of unique forms or configurations imparted to the vitreous envelope of the lamp.

Accordingly the object of the invention is to provide a new and improved double-groove discharge lamp aving a higher efliciency or lumen per watt characteristic than heretofore.

Another object of the invention is to provide a double groove lamp of unique form or configuration in the merging or oblique sections through which the discharge or plasma channel is transferred from one side to the other of the envelope.

Another object of the invention is to provide a double groove lamp which will permit a higher loading at a given efliciency within a given linear length of lamp.

In accordance with the invention, the indentations or sections of groove are relatively short and closely spaced; they alternate on opposite sides of the envelope and are relatively open so as to avoid as much as possible trapping of light on them. They are so disposed that the wall-to-wall spacing in them merging or oblique sections is relatively narrow and causes substantial confinement of the plasma. Preferably the wall-to-wall spacing in the merging sections is no greater than in the groove sections. This results in a discharge or plasma channel of approximately uniform width or thickness when measured along the medial line of the grooves, and which extends in sinuous or zigzag fashion substantially the entire length of the lamp. This configuration has the advantage of providing a more uniform constriction of the plasma. At the same time the zigzag path is substantially longer than the straight line inter-electrode path would be and causes a higher voltage drop across the lamp entailing higher efficiency and permitting higher loading for a given lumen per watt ratio.

According to another feature of the invention, further improvements in eificiency or lumen per watt ratio may be achieved by providing, in the slanting or oblique sections, a cross section generally like that of a spool of thread having substantially straight sides. This increases the efliciency of conversion of electrical energy into light within the merging sections.

For a more detailed description of the invention, attention is now directed to the following description and accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

' of the lamp QfFig. 1 taken in plane 3-3.

In the drawing:

Fig. 1 is a side elevation view of a discharge lamp embodying a preferred form of the invention, portions of the envelope being cut away to shorten the figure.

Fig. 2 is a plan view of the lamp of Fig. 1, part of the envelope being shown in section.

Fig. 3 is a transverse section through a grooved portion Figs. 4a and 4b are oblique sections taken approximately normal to the side walls in a merging portion of the lamp in the plane 44 of Fig. 1. Fig.4b'shows the improved straight-sided spool contour in accordance with the invention. a

Referring to the drawing and more particularly to Figs. 7

1 and 2, there is shown a fluorescent lamp of the lowpressure positive column type comprising an elongated vitreous envelope 1. -The envelope is provided with circular or round tube ends 2, 2 which are annularly reduced or shouldered at their extremities for securing thereto bases 3, 3. In order to foreshorten the figure, intermediate portions'of the lamp have been cut away. As may be seen through the cut-out at one'endof the lamp, an electrode mount or stem 4 is sealed into each end. A pair gases of group of the periodic table at a low pressure, for instance argon at a pressure of 0.5 to 5 millimeters of mercury or alternativelya mixture of argon and not over approximately 50% neon in this range of pressure. Increasing the proportion of lower atomic weight inert gas such as neon, and all the more so in the case of helium, or reducing the starting gas pressure permits a gain in loading capacity or in efficiency. However the gain'is achieved in increasing degree at the expense of life inasmuch as the cathodes begin to disintegrate inordinately rapidly. "Ina preferred embodiment representing a studied choice between life and performance characteristics ineluding efliciency andloading capacity, argon at a pres- 1 sure of 1.4 millimeters of mercury is used. The mercury vapor is provided by a small quantity of mercury exceedstarting gas or mixture of one or more of the inert rare tance A between corresponding points in adjacent grooves on opposite sides of the envelope is comparable to the maximum diameter of the envelope; it is not over greater nor under 33% less and is preferably approximately equal to it. For instance in a lamp of 2 /8" diameter, the groove throw may be approximately 2 /8". This assures an appreciable increase in the effective length of the plasma channel inasmuch as it forms a continuous zigzag path substantially the entire length of the envelope. This is equivalent to lengthening the envelope although the physical length of course remains unchanged.

According to another feature of the invention most clearly illustrated in Fig. 3, the grooves are proportioned, relative to the envelope diameter, 'to be comparatively open. If the grooves are too deep or narrow, light tends to be trapped in them. This undesirable result is substantially avoided by selecting proportions giving an angle B, measured between two straight lines starting from the midpoint of the bottom wall of the groove at 12 and diverging to the edges or rails at 13 and 14, which is at least 90. In the illustrated embodiment, angle B, which may be referred to as the groove aperture angle, is approximately 100.

The ends 15, 16 of adjacent interposed grooves on opposite sides of the envelope are sufliciently close together to form desirable constricted merging or intermediate zones 17 between them. Preferably, as shown in Figs. 2 to 4, the wall-to-Wall spacing C in merging zones 17 is not greater than the wall-to-wall spacing D in grooved zones 18 measured between the bottom wall of the groove at 12 and the curved outer wall of the envelope at 19. Thus there is provided a zigzag path which the discharge or plasma is constrained to follow from one end of the lamp to the other, and in which the discharge is generally constricted as measured in the medial diametral plane through the grooves. This substantially improves the efficiency or lumen per watt ratio of the lamp inasmuch as it causes the efliciency to be almost ashighin the merging zones as it is in the grooved zones. 7 There results a very sub stantial improvement in performance or loading capacity over the single-groove bridged lamp as exemplified by that commercially designated 96 PG 17 In the illustrated embodiment, dimension D is approximately 0.9 inch and dimension C is 0.7 to 0.75 inch, that is C is 78% to 83% of D. In general, C should be not more than 150% of D in order to obtain confinement of the plasma in the merging zones to any worthwhile extent,

- and preferably should be not more than 100% of D. In

ing in' amount the quantity vaporized during operation of the lamp wherein the mercury vapor may exert a partial pressure in the range of 1 to 20 microns. The optimum pressure for maximum efiiciency of generation of mercury resonance radiation at 2537 A. in this particular lamp'is approximately 6 microns. A phosphor coating indicated at 8 on the inside of the envelope, and which may consist of alkaline earth halophosphates, converts the 2537 A; radiation into visible light.

The envelope is sometimes described as multiple or double grooved. It is provided with spaced indentations or're-entrant portions 10, 11 on diametrically opposite sides giving it a dimpledor crenelated appearance. Otherwise viewed, the indentations 10, 11 may be consideredto be short sections of a longitudinal groove alternating on vides in addition good lamp etficiency by reason of the.

higher electron velocities and reduced elastic collision losses. These characteristics are'due ofcourse to the more rapid diflt'usion of photons, electrons, and mercury ions to the walls as previously'me'ntioned.

' According'toa feature of the invention most clearly illustrated in Fig; 2, the grooves are short and relatively closely spaced. The groove throw,that is the axial disorder to avoid instability, C should be not less than 60% of D: otherwise the discharge tends to be unstable and to flick back and forth from one side to the other in the merging zones. The chosen proportions wherein C is approximately 80% of D allow for manufacturing'tolerances and avoid instability while achieving a very worthwhile degree of plasma confinement or constriction.

The narrower wall-to-wall spacing in the merging zones also causes sharper bends in the discharge path or plasma a in its alternations from one side of the lamp to the other and thereby assists in further lengthening the effective length of the positive column. This increases the loading capacity or lamp wattage going into the positive column relative to fixed electrode losses, thereby increasing the efficiency. Both the confinement or constriction of the plasma throughout the lamp and its increase in length by reason of the zigzag path which it is constrained to follow are effective in raising the voltage drop across the lamp for a given wattage input. The lamp therefore operates at a higher voltage and lower current than would otherwise be the case and this situation is advantageous for reducing the copper and iron requirements of the ballast. The lamp thus permits the design of a ballast having not only a lower first cost but also a lower operating cost through reduced ballast losses.

. a In accordance with another feature of the invention, we

have found that a further appreciable improvement in performance may be achieved by providing in the oblique merging zones a cross section which may be described as a straight sided spool configuration as distinguished from a curved sided spool. The improved cross section is illustrated in Fig. 4b wherein the central portions of the vertical side walls 15b and 16]) are substantially straight or flat. Fig. 4a illustrates for purposes of comparison a cross section wherein the central portions of side walls 15a and 16a are curved.

Irrespective of which spool configuration is used in the merging section, the ionized discharge column or plasma will assume a generally oval configuration and will not effectively penetrate into the corner recesses at 20a or 20b. In both cases the limits of expansion of the plasma oval are determined by the nearest portion of the boundary walls, that is upper and lower walls 21, 22 and vertical side walls 15a, 16a in Fig. 4a or 15b, 16b in Fig. 417. However for a given minimum wall-to-wall spacing between the vertical side walls, the plasma oval will in general conform more closely to the walls in the straightsided spool configuration of Fig. 4b. Otherwise stated, in the configuration shown in Fig. 4b, the plasma may be said to spread itself closer to the walls. This is the desired condition for raising the efficiency by confining or constricting the discharge inasmuch as difiusion of photons, electrons and mercury ions to the walls occurs more rapidly with resulting improvements in electrical efficiency and lumen per watt ratio.

A lamp with the improved groove configuration illustrated in the drawing may be formed by the same technique described in application Serial No. 578,772 of Eugene Lemmers et al., filed April 17, 1956, now Patent No. 2,916,645, entitled Tubular Lamp Envelopes, and assigned to the same assignee as the present invention. According to this technique, the lamp envelope is selectively heated in the zones where the grooved panels are to be formed until the glass becomes suitably plastic. A plunger or pressing bar provided with suitable ridges or protuberances corresponding generally to the grooves required to be formed is then pressed down against the envelope. The ridges have the desired radius of curvature of the bottom of the grooves, for instance =7 in a transverse plane and 1" in a longitudinal plane for forming a lamp of the T-l7 size having a nominal diameter of 12%. In order to provide the straight-sided spool contour in the cross section of the merging portions, the slanting ends of the ridges on the plunger or pressing bar are formed substantially flat instead of curved.

The lamp is desirably provided with at least one and preferably two asymmetric grooves to serve as a mercury vapor pressure control center as taught and claimed in copending application Serial No. 734,5 69 of John O. Aic er and Eugene Lemmers filed concurrently herewith, entitled Grooved Lamp Vapor Pressure Control, and assigned to the same assignee as the present invention. The two asymmetric grooves are preferably located near the longitudinal center of the lamp and three groove throws apart as at 10' and 11' in Figs. 1 and 2. in the asymmetrically grooved panels, the groove is deeper and the rail portion on one side of the groove is wider than on the other. The discharge plasma then displaces itself towards the Wider rail side and away from the narrower rail side. The narrower rails then operate at a cooler temperature than the remainder of the envelope. The narrower rail portion which is located lowermost is more eifectively cooled by convection fiow of air. In order to have a non-polarized lamp for mounting with the grooves facing the sides and which can be rotated at will 180 in either direction, the two asymmetric grooves are offset in opposite directions relative to the diametral medial plane through the grooves. This provides a pair of narrower rail portions on diametrically opposite sides of the lamp so that one of the pair is always located lowermost and that one serves as the effective mercury vapor pressure control center.

A preferred lamp construction embodying the various features of the invention and which may be designated 96 MPG 17 (multiple power groove), has a nominal length of 8', diameter of 2 /8, and a groove throw of approximately 2 /8. This results in a total of 40 grooves or indentations, 20 grooves to a side. Performance measurements indicate average lamp load-ing 225 watts, lamp current 1500 milliamperes, lamp voltage 178 volts, and lumen output 15,500 lumens with the mercury vapor pressure control center at 40 C., the lumen per watt figure being 68.9. Thus by comparison with the single groove bridged lamp designated 96 PG 17 described earlier, it is seen that the present double grooved 96 MPG 17 lamp embodying the invention provides an increase in loading capacity of 12.5% (200 watts to 225 Watts), coupled with a gain in efficiency of approximately 6% (65 lumens per watt to 68.9 lumens per watt). 'In addition, the envelope has greater resistance to implosion so that the wall thickness may be reduced from .070" to .050" for the same margin of safety.

While a certain specific embodiment of the invention has been illustrated and described in detail, it is intended as illustrative and not as limitative of the invention. The scope of the invention is to be determined by the following claims which are intended to cover any modifications coming within its true spirit and scope.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and a metal vapor producing resonance radiation, said envelope having an outer wall of generally circular section and a plurality of relatively short longitudinally extending grooves alternating on opposite sides of the envelope and resulting in a generally kidney-shaped cross section in the grooved portions of the envelope, the groove throw as measured between corresponding points in adjacent grooves on opposite sides of the envelope being comparable to the envelope diameter in order to confine the plasma in a substantially lengthened zigzag discharge path.

2. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and a metal vapor producing resonance radiation, said envelope having an outer wall of generally circular section and a plurality of relatively short longitudinally extending grooves alternating on opposite sides of the envelope to confine the plasma in a substantially lengthened zigzag discharge path, the cross section of said envelope being generally kidneyshaped in the grooved portions, said grooves having slanting end walls and an aperture angle measured between the bottom of the groove and the rails on either side not less than 3. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and a metal vapor producing resonance radiation, said envelope having an outer wall of generally circular section and a plurality of relatively short longitudinally extending grooves alternating on opposite sides of the envelope and resulting in a generally kidney-shaped cross section in the grooved portions of the envelope, the groove throw as measured between corresponding points in adjacent grooves on opposite sides of the envelope being comparable to the envelope diameter in order to confine the plasma in a substantially lengthened zigzag discharge path, said grooves having slanting end walls and an aperture angle measured between the bottom of the groove and the rails on either side not less than 90.

4. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed Y confine the plasma in a substantially lengthened l7 into opposite ends and containing an ionizable medium including an inert starting gas and a metal vapor producing resonance radiation, said envelope having an outer wall of generally circularsection and a'plurality of rela- 'tively short longitudinally extending grooves alternating on opposite sides ofthe envelope and resulting in a generally kidney-shaped cross section in the grooved portions of the envelope, said grooves having slanting end walls and being disposed along the length of the envelope to provide merging sections with a relatively narrow wall-to-wall spacing measured between said slanting end walls causing substantial constriction of the plasma therein.

5. A low pressure electric discharge lamp comprising an elongated *vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and a metal vapor pro ducing resonance radiation, said envelope having an outer wall of generally circular section-and a plurality of relatively short; longitudinally extending grooves alternating "on opposite sides of the envelope and resulting in a genwall spacing measured between said slanting end walls causing substantial constriction of the plasma therein, the

groove throw as measured between corresponding points 4 in adjacent grooves on opposite sides of the envelope being comparable to the envelope diameter in order to zigzag discharge path. 7 a 1 6. A low pressure electric discharge lamp'comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and a metal vapor producing resonance radiation, said envelope having an outer wall of generally circular section and a plurality of relatively short longitudinally extending grooves alternating on opposite sides of the envelope and resulting in a generally kidney-shaped cross section-in the grooved portions of the envelope, said grooves having slanting end walls and being disposed along the length of the envelope to provide merging sections with a wall-to-wall spacing measured between said slanting end walls in the range of 60% V ternally with a phosphor and having electrodes sealed into opposite ends andcontaining an ionizable medium ineluding an inert'starting gas and mercury vapor producing 2537 A. resonance radiation for exciting said phosphor, said envelope having an outer wall of generally circular section and a plurality of relatively short longitudinally extending grooves alternating on opposite sides of the envelope and resulting in a generally kidney-shaped cross section in the grooved portions of the envelope, said grooves having slanting end walls and being disposed along the length of the envelope to provide merging sections with a wall-to-wall spacing measured between said slanting end walls in the range of 60% to 100% of the wall-to-wall spacing in the grooved portions measured between the bottom of the groove and the circular outer wall of the envelope, whereby to provide a zigzag channel for the electric discharge plasma extending the length of the envelope and having an approximately uniform wall-towall spacing measured in the medial plane of the grooves. 8. A low pressure electric discharge fluorescent lamp of the double groove type comprising an elongated vitreous envelope coated internally with a phosphor and having electrodes sealed into opposite ends and containing an ionizable medium including an inert startingfgas and mercury vapor producing 25 37 A. resonanceradiation for exciting sa-id phosphor, said envelope having an outer wall of generally circular section and a plurality of relatively short longitudinally extending grooves alternating on opposite ured between corresponding points in adjacent grooves on opposite sides of the envelope being comparable to the envelope diameter in order to confine the plasma in a zigzag channel having an approximately uniform wall-to-' wall spacing measured in the medial plane of the grooves and of substantially greater length than the straight line interelectrode distance.

9. A lamp as in claim 8 wherein the groove throw is approximately equal to the envelope diameter.

10. A lamp as in claim 8 wherein said wall-to-wall spacing in the merging sections is approximately 80% of said wall-to-wall spacing in the grooved portions and wherein the groove throw is approximately equal to the envelope diameter.

11. A lam-p as in claim 8 wherein the aperture angle measured between the bottom of the grooves and the rails on either side is not less than 90%.

12. A lamp as in claim 8 wherein said wall-to-wall spacing in the merging sections is approximately 80% of said wall-to-wall spacing in the grooved portions and wherein the groove throw is approximately equal to the envelope diameter and wherein the aperture angle measured between the between the bottom of the grooves and rails on either side is approximately 100..

13. A low pressure electric discharge lamp conprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and a metal vapor producing resonance radiation, said envelope having an outer wall of generally circular section and a plurality of relatively short longitudinally extending grooves alternating on opposite sides of the envelope,

- said grooves being generally rounded in cross section whereby to provide in said grooved portions a generally kidney-shaped cross section for the electric discharge plasma, and said grooves having substantially flat slantinert star-ting gas and a metal vapor producing resonance radiation, said envelope having an outer wall of generally circular section and a plurality of relatively short longitudinally extending grooves alternating on opposite sides of the envelope, said grooves being generally rounded in cross section whereby to provide in said grooved portions a generally kidney-shaped cross section with an aperture angle measured between the bottom of the groove and the rails on either side not less than and said grooves having substantially flat slanting end walls in order to provide merging portions between the grooved portions wherein the cross section of the discharge space is generally spool-shaped with substantially straight side walls, whereby to achieve greater electrical efiiciency and a higher lumen per watt ratio in said merging portions.

15. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and a metal vapor producing resonance radiation, said envelope having an outer wall of generally circular section and a plurality of relatively short longitudinally extending grooves alternating on opposite sides of the envelope, said grooves being generally rounded in cross section whereby to provide in said grooved portions a generally kidneyshaped cross section with an aperture angle measured between the bottom of the groove and the rails on either side not less than 90, said grooves having substantially flat slanting end walls whereby to provide merging portions between the grooved portions wherein the cross section of the discharge space is generally spool-shaped with substantially straight side walls, and said grooves being disposed along the length of the envelope with a groove throw as measured between corresponding points in adjacent grooves on opposite sides of the envelope comparable to the envelope diameter whereby to confine the plasma in a zigzag channel of substantially greater length than the straight line inter-electrode distance.

16. A low pressure electric discharge fluorescent lamp of the double groove type comprising an elongated vitreous envelope coated internally with a phosphor and having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and mercury vapor producing 2537 A resonance radiation for exciting said phosphor, said envelope having an outer wall of generally circular section and a plurality of relatively short longitudinally extending grooves alternating on opposite sides of the envelope, said grooves being generally rounded in cross section whereby to provide in said grooved portions a generally kidney-shaped cross section with an aperture angle measured between the bottom of the groove and the rails on either side not less than 90, said grooves having substantially flat slanting end walls whereby to provide merging portions between the grooved portions wherein the cross section of the discharge space is generally spool-shaped with substantially straight side walls, said grooves being disposed along the length of the envelope to provide a wall-towall spacing measured between said flat slanting end walls in the range of to 100% of the wa'll-to-wall spacing in the grooved portions measured between the bottom of the groove and the circular outer wall of the envelope, the groove throw as measured between corresponding points in adjacent grooves on opposite sides of the envelope being comparable to the envelope diameter in order to confine the plasma in a zigzag channel having an approximately uniform wall-to-wall spacing measured in the medial plane of the grooves and or" substantially greater length than the straight line 7 inter-electrode distance in order to achieve a greater electrical efiiciency and a higher lumen per watt ratio.

17. A lamp as in claim 16 wherein said groove throw is approximately equal to the envelope diameter.

18. A lamp as in claim 16 wherein the wall-to-wall spacing in said merging portions is approximately of the wall-to-wall spacing in said grooved portions.

19. A lamp as in claim 16 wherein said aperture angle is approximately 20. A lamp as in claim 16 wherein said groove throw is approximately equal to the envelope diameter, wherein the wall-to-wall spacing in said merging portions is approximately 80% of the wall-to-wall spacing in said grooved portions, and wherein the aperture angle is approximately 100.

References Cited in the file of this patent UNITED STATES PATENTS 1,974,888 Barclay Sept. 25, 1934 2,317,265 Foerste et al. Apr. 20, 1943 FOREIGN PATENTS 860,383 Germany July 8, 1949 906,245 Germany Mar. 11, 1954 

